A TSM sensor is a device which generates mechanical vibrations from an electrical signal and uses these vibrations to detect and/or quantify particular chemical or biochemical substances present in a medium surrounding the sensor (the analyte). Acoustic energy is stored and dissipated both in the device itself, and through interfacial coupling, in a surrounding liquid medium. By coating the sensor with one or more layers of a substance which interacts with the analyte, the energy storage and transfer processes change when the interaction occurs. This changes the acoustic resonance of the sensor, which can be observed by measuring the electrical impedance of the sensor. The applicants have published several papers in this field and they are listed as follows:                1.) F. Ferrante, A. L. Kipling and M. Thompson, “Molecular Slip At The Solid-Liquid Interface Of An Acoustic Wave Sensor”, J. Appl. Phys. 76(6):3448–3462, 1994;        2) G. L. Hayward and M. Thompson, “A Transverse Shear Model Of A Piezoelectric Chemical Sensor”, Amer. Inst. Physics 83(40:2194–2201, 1998;        3) Cavic B. A. et al., “Acoustic Waves And The Real-Time Study Of Biochemical Macromolecules At The Liquid/Solid Interface”, Faraday Discuss. 107:159–176, 1997;        4) H. Su and M. Thompson, “Rheological And Interfacial Properties Of Nucleic Acid Films Studies By Thickness-Shear Mode Sensor And Network Analysis”, Can. J. Chem. 74:344–358, 1996.        
There are several mechanisms whereby a TSM sensor responds to chemical change on its surface when it is immersed in a liquid. Surface mass deposition occurs when the analyte binds to the receptor on the sensor surface. This increases the storage of acoustic energy through the inertia of the added mass. Acoustic energy may also be stored through the elastic deformation of a coating on the surface. The elasticity of the coating may also change when the analyte binds to the receptor coating. These energy storage modes determine the resonant characteristics of the sensor which can easily be measured electrically. These processes are well known. Examples of piezoelectric sensors are described, for example in U.S. Pat. Nos. 5,374,521 and 5,658,732.
Viscous loading occurs when acoustic energy is transferred to the liquid. Some of the acoustic energy is stored by the inertia of the fluid moving with the sensor surface and can be transferred back to the sensor, but acoustic energy is also dissipated by internal friction within the fluid. The viscous loading effect is also well known, however in the current use of this effect, the transfer of acoustic energy at the surface is considered to be perfect, that is, there is no slip between the sensor surface and the adjacent fluid molecules.
The current practice is based on the well known Butterworth—van Dyke model of a piezoelectric resonator which consists of a resistor, inductor and capacitor in series, all in parallel with another capacitor. The series arm of this network is called the motional arm. Further details of this model and the calculation of the following parameters may be found in the above paper entitled “Rheological and Interfacial Properties of Nucleic Acid Films Studies by Thickness-Shear Mode Sensor and Network Analysis”.
Motional Inductance
The motional inductance, LM, represents the inertial energy stored by the sensor. It depends on the mass of the TSM sensor as well as the mass of material (the analyte) added to the surface. Since liquid coupled to the surface can store and return acoustic energy, LM is also dependent on the viscosity of the liquid.
Motional Resistance
The motional resistance, RM, is intrinsically related to the energy dissipated by the sensor.
Accordingly, any imposition of material (or loss of material) that has a viscous property or changes in the viscosity of the liquid will result in a change in the energy dissipation and hence RM.
Motional Capacitance
The motional capacitance, CM, represents the elastic energy stored by the sensor. The absorption or chemical binding of the analyte to the coating can have a large effect on the viscoelastic properties of the coating. Depending on the thickness, an added (or removed) layer of material may change the elasticity of the sensor and thus affect CM. Although most fluids are considered to be viscous, at the high frequencies used in piezoelectric quartz sensors, the liquid may also have elastic properties.
Static Capacitance
The static capacitance CO represents the dielectric constant of the quartz, but includes that of the medium through the electric field. Charge interactions between the analyte and the sensor coating will affect this value.